The secondary carbocation shown rearranges by a single 1,2-shift to a more stable cation. Which structure shown is that cation?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: A

The cation shown is the 3-methylbutan-2-yl cation. A 1,2-hydride shift from the neighbouring tertiary C-H moves the charge to the more substituted carbon, giving the 2-methylbutan-2-yl (tert-amyl) cation. This tertiary cation has more alkyl groups for hyperconjugation and is more stable than the secondary cation, whereas the primary cations are less stable.

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